Especially in frequency bands now coming up for mobile broadband use—like for example the 700 MHz frequency bands of long term evolution (LTE) systems in the U.S. and in Europe—there are often Digital TV, Mobile TV, or MediaFLO spectrum allocations next to an LTE spectrum allocation. For the remainder of this description, MediaFLO is taken as an example for a strong extra-system interferer.
MediaFLO is a technology to transmit data to portable devices such as cell phones and PDAs, used for mobile TV. In the United States, the MediaFLO system uses the frequency spectrum 716-722 MHz.
Hence, on top of the intra-system (inter-cell) interference inside the LTE system, MediaFLO base station transmitters interfere with the LTE base station receiver. This type of interference can be characterized as spatially and spectrally coloured. The MediaFLO interference power depends on the antenna tilting, transmission power, and the transmit filter of the MediaFLO base station as well as on the LTE antenna tilting and the guard band between the MediaFLO transmit and the LTE receive channels. At average the MediaFLO interference power at the spectrally closest LTE Physical Resource Block (PRB) may be up to 50 dB above the noise level for tight coexistence situations, and up to 30 dB in realistic coexistence situations, while typical intra-system average interference power is expected to be 5 to 10 dB above the noise level.
Without Uplink Inter-Cell Interference Coordination, intra-system interference for a full Frequency Reuse 1 scenario is (at average) equal for all PRBs of the LTE channel allocation while adjacent channel leakage from MediaFLO creates high interference on PRBs spectrally close to the MediaFLO channel and low interference on PRBs spectrally further away from the MediaFLO channel.
FIG. 1 illustrates a simulation of interference from adjacent channel leakage into a 10 MHz LTE channel from a very strong extra-system interferer like MediaFLO. The frequency scale is indicted relative to the frequency of the interferer. Line 11 marks the noise floor (reference bandwidth in this case is 500 KHz); line 12 marks the average interference power from the extra-system interferer, and line 13 marks the interference rise on the LTE base station receiver as the orthogonal frequency division multiplex (OFDM) SINC( ) functions' side lobes take-in the interferer's leakage power even on physical resource blocks (PRBs) spectrally further away from the interferer.
In an LTE Frequency Reuse 1 network interfered from MediaFLO, the application of Interference Rejection Combining on a full 10 MHz or even 20 MHz channel with 2, 4, or even more LTE receive antennas creates such a computational effort potentially exceeding available processing capacity of the LTE base station.